Add support for creating new window

[dvcrn]

Things to do:

• [ ] open-file event should open a new window
• [ ] Ctrl/Cmd + N should create a new window
• [ ] Ctrl/Cmd + O should open the file in the current frontmost window
• [ ] update notifications should appear in the current frontmost window
• [ ] dev tools should open in the front most window
• [ ] reload should reload the front most window
• [ ] exit / quit should check all open windows for unsaved changes

Things to investigate:

• will our IPC implementation work with multiple windows?

[SeriousM]

:+1: